Commercial roofs are rated based on fire resistance. The fire rating is based on the ability of the roof's surface to withstand one or more tests. To obtain a Class A over wood deck fire rating, a roofing system must pass ASTM E 108 (hereinafter, this is simply referred to as a Class A fire rating). In this test, combustible material is placed on the roof surface, ignited and allowed to burn. If the roof's structure does not sustain a flame, meets structural integrity requirements after the test is complete, and does not have holes larger than a certain size after the test, the roof system passes the test. A comparable test method is UL 790. Built-up roofs which are formed from bituminous material can pass this test by using multiple layers of a glass matting. The glass mats provide an inorganic barrier layer that prevents the fire from extending through the surface of the built up roof.
Membrane roofs which are formed from polymeric sheet membranes and, in particular, non-bituminous polymeric sheets, can achieve a UL Class A over wood deck rating. But the roof system is much more expensive. It requires a fireproof material such as gypsum board or fiberglass facers over an intumescent coating. The added expense often makes such roofs cost-prohibitive.